The present invention relates principally to a plasma generating apparatus used in fabricating semiconductor devices, liquid crystal display panels and solar cells and particularly in processes of etching, ashing, thin film growth by chemical vapor deposition (CVD), sputtering, surface modification and chamber cleaning, and more specifically to a plasma generating apparatus of generating plasma by applying high-frequency power to gas for plasma generating, and particularly to technology for retarding thermal deterioration of a discharge tube. However, an application subject of a plasma generating apparatus in accordance with the present invention is not intended to be limited to the fabrication of semiconductor devices and is widely applicable for any desired objects.
In FIG. 18, reference numeral 101 indicates an insulating discharge tube, numeral 102 an insulating cooling medium flow tube, numeral 103 an upper flange for supporting a discharge tube, numeral 104 a lower flange for supporting a discharge tube, numeral 104a a cooling medium inlet, numeral 103a a cooling medium outlet, numeral 105 a gas introduction flange, numeral 105a a gas induction port for plasma, numeral 106 a flange for mounting to a device, numeral 106a a plasma discharge port, numeral 107 an induction coil, numeral 108 a high-frequency power supply, numerals 109 to 111 O-rings, numeral 112 a plasma generating space and numeral 113 a cooling medium flow space.
The outer periphery of the discharge tube 101 formed cylindrically is enveloped with the cooling medium flow tube 102 and the upper flange 103 for supporting a discharge tube is fitted externally to be fixed to the top end of the discharge tube 101 and the cooling medium flow tube 102 with O-rings 109, 110, respectively, interposed, and the lower flange 104 for supporting a discharge tube is fitted externally to be fixed to the bottom end of the discharge tube 101 and the cooling medium flow tube 102 with O-rings for vacuum-sealing 109, 110, respectively, interposed, and a double tube structure is constructed through such a fitting construction. In this double tube structure, the discharge tube 101 and the cooling medium flow tube 102 are coaxial, and the annular cooling medium flow space 113 is formed between both. The lower flange 104 for supporting a discharge tube is provided with the cooling medium inlet 104a communicating with the cooling medium flow space 113, and the upper flange 103 for supporting a discharge tube is provided with the cooling medium outlet 103a communicating with the cooling medium flow space 113. The gas introduction flange 105 which almost blocks the top end opening of the discharge tube 101 is made to abut on and fixed to the upper flange 103 for supporting a discharge tube with the O-rings 111 for vacuum-sealing interposed. The gas introduction port for plasma 105a is provided along a central axis of the gas introduction flange 105 in a state of communicating with the plasma generating space 112 of the discharge tube 101. The flange 106 for mounting to a device is made to abut on and fixed to the lower flange 104 for supporting a discharge tube in a state of communicating with the plasma generating space 112 of the discharge tube 101 with the O-rings 111 interposed. The induction coil 107 is wound on an outside of the outer cooling medium flow tube 102 constituting the double tube structure, and the induction coil 107 is connected to the high-frequency power supply 108. Further, an impedance matching device, not shown, is interposed between the induction coil 107 and the high-frequency power supply 108.
A plasma generating apparatus constructed as described above is used in a state of being attached to a plasma processing chamber, not shown, at the flange 106 for mounting to a device. Interior space of the plasma processing chamber and the plasma generating space 112 of the plasma generating apparatus is evacuated by evacuating the plasma processing chamber of air. Then, gas for plasma generation (discharging gas) is supplied from the gas introduction port for plasma 105a toward the plasma generating space 112 and high-frequency power is supplied to the induction coil 107 by actuating the high-frequency power supply 108, and a high-frequency electromagnetic field is generated in the plasma generating space 112. Simultaneously, a cooling medium is supplied from the cooling medium inlet 104a located on the lower side, is moved upward in the cooling medium flow space 113 and is discharged from the cooling medium outlet 103a. 
The gas for plasma generation flows into the plasma generating space 112, and plasma ignition due to high-frequency discharge occurs in a small region of the flowing gas by action of the high-frequency electromagnetic field upon the gas for plasma generation. Resulting from the plasma ignition occured in a small region, plasma generation is spread almost across the whole of the gas flowing in the plasma generating space 112. The gas which is ionized to plasma in the plasma generating space 112 in this manner, i.e., plasma flows from the plasma discharge port 106a of the flange 106 for mounting to a device into the plasma processing chamber, not shown, and performs plasma processing, such as etching and ashing for semiconductor wafer and liquid crystal substrate, in the plasma processing chamber.
The plasma which is generated in the plasma generating space 112 becomes a high temperature. The discharge tube 101 forms the plasma generating space 112 which is a space for ionizing the gas for plasma generation being introduced from the gas introduction port for plasma 105a to plasma. Accordingly, the discharge tube 101 is raised in temperature due to contact with plasma of a high temperature. To limit the temperature rise within a predetermined range, heat exchange is conducted by flowing the cooling medium through the cooling medium flow space 113 surrounding the outer periphery of the discharge tube 101.
Though the discharge tube 101 is cooled as described above, it is inevitable that temperature difference occurs between the face of the inner circumference of the discharge tube 101, exposed to plasma of a high temperature, and the face of the outer periphery of the discharge tube 101, contacting with the cooling medium. Further, generated plasma of high temperature is localized in some cases, and so temperature difference is developed in the direction of a tube axis and in the circumferential direction in the discharge tube 101 as well. Thermal deformation is produced in the discharge tube 101 resulting from such temperature difference.
In a plasma generating apparatus of this sort of high-frequency discharge of inductive coupling type in which induction coils are arranged on an outer periphery of a discharge tube for a discharge tube for flowing the gas for plasma generation, the following conditions are required. That is, since a high-frequency electromagnetic field should be formed in the plasma generating space 112 of the discharge tube 101 by high-frequency power supplied from the induction coils 107 arranged outside of the discharge tube 101, the discharge tube 101 should be made from material other than a conductor, i.e.,an insulator (inductor) in order not to form electromagnetic shielding. The cooling medium flow tube 102 located at an outer periphery of the discharge tube 101 should also be of insulator. As such an insulator, there are, for instance, quartz, high purity alumina, aluminum nitride, glass and sapphire. Cooling medium flowing through the cooling medium flow space 113 is preferably a substance which does not absorb high-frequency power applied as far as possible, as well.
As described above, thermal deformation tends to be easily produced in the discharge tube 101, but since the discharge tube 101 is made from insulator, it has low thermal conductivity and the insulating discharge tube 101 experiences high temperature of the inner surface thereof and is apt to cause thermal deterioration in spite of cooling.
In addition, the discharge tube 101 has a large pressure difference between the outside atmosphere and the inside of the wall of the discharge tube since the plasma generating space 112 thereof is evacuated.
Further, though gas for plasma generation introduced into the plasma generating space 112 varies with a use and a condition, in some cases highly reactive plasma is generated depending on a gas property and there is possibility that the discharge tube 101 is subject to corrosion.
Further, an insulator is generally low in strength in comparison with metal material.
Due to such a high temperature, a temperature difference, a pressure difference and corrosion, there is a possibility that failures such as cracks and fractures are generated in the discharge tube 101, which is made from an insulator and relatively low in the strength, when in use for a long time. If the insulating discharge tube 101 should fail, and vacuum-sealed conditions are broken and it is not capable of producing a vacuum condition of the plasma generating space 112, plasma generation itself becomes impossible. And, if the insulating discharge tube 101 fails, the cooling medium escapes and the escaped cooling medium flows into a plasma processing chamber unexpectedly and possibly contaminates substances to be processed such as semiconductor wafer.
There is also a plasma generating apparatus of a capacitive coupling type other than such an inductive coupling type. This apparatus is one in which a pair of curved-shaped electrodes are opposed on a face of the outer periphery of a discharge tube made from an insulator such as glass, and high-frequency power is applied to both electrodes. A high-frequency electromagnetic field is produced in a plasma generating space within a discharge tube, discharge is developed by ionizing gas for plasma generation, and therefore plasma is generated. Also in this case, as the discharge tube, an insulating discharge tube is used, and therefore there is the same problem as the above.
Further, the discharge tube 101 made from insulator prepared with quartz, high purity alumina, aluminum nitride, glass and sapphire is considerably high in its initial cost.
Accordingly, it is a principal object to provide a plasma generating apparatus to prevent failure such as cracks and fractures and leakage of cooling medium due to thermal deterioration of a discharge tube.
The present invention of a plasma generating apparatus intended to attain the object described above uses metal material as component material of a discharge tube in place of insulator of prior art. The reason for this is that metal material has a higher resistance against heat and a higher physical strength compared with insulator.
However, it is insufficient only to replace the material. The reason for this is that when the whole discharge tube has been simply composed with metal material, high-frequency power generated by an induction coil located on an outer periphery of a discharge tube cannot enter into the interior of metal material, i.e., a discharge tube due to the effect of electromagnetic shielding by metal material, i.e., a conductor. In other words, high-frequency power cannot be provided from an induction coil located on an outer periphery of a discharge tube to a plasma generating space within a discharge tube.
There is a plasma generating apparatus of a capacitive coupling type, rather than an inductive coupling type, utilizing high-frequency power. In the capacitive coupling type, when the whole discharge tube is composed of metal material, there is a problem of how to construct and to locate a pair of electrodes. When a pair of electrodes are located within a discharge tube of metal material, each electrode is required to be located in a state of being insulated and isolated from a discharge tube of metal material. When a pair of electrodes are located within a discharge tube, these electrodes become an obstruction against the flow of gas/plasma. Further, since a pair of electrodes is required, this leads to complication of the structure and increase in cost.
Thus, the object cannot be attained by a simple idea that component material of a discharge tube is only replaced with metal material.
Consequently, in the present invention, when a discharge tube is composed of metal material which is excellent both in thermal resistance and in physical strength compared to an insulator, it is noted that the discharge tube is a conductor and it is functionally the same as an electrode in the fact of being a conductor. That is, it is feasible to use a discharge tube both as itself and as an electrode, conversely, the feasibility of using an electrode both as itself and as a discharge tube exists.
In view of the above, a plasma generating apparatus of a first embodiment of the present invention is one, wherein each discharge tube element, of a discharge tube having a plurality of sections in a direction of a tube axis, is fabricated from metal materials and the discharge tube elements of metal material are connected hermetically to each other with an insulator interposed and a discharge gap is formed between the discharge tube elements. The foregoing plasma generating apparatus is constructed so to enable plasma ignition at foregoing discharge gap by applying high-frequency power.
Accordingly, a discharge tube is obtained by dividing a discharge tube at one point or a plurality of points in the direction of a tube axis thereof by cutting across the direction of the tube axis. The number of the discharge tube elements is two or more. A plurality of the discharge tube elements are lined in the direction of the tube axis and combined, to thereby construct a discharge tube. Each individual discharge tube element is fabricated from metal materials. When a plurality of the discharge tube elements of metal material are lined in the direction of the tube axis and combined, connecting the discharge tube elements to each other in a state of direct contacting causes the whole of the discharge tube obtained as a result of being connected to become a single conductor, so that the discharge tube cannot be used both as itself and as an electrode. Further, there is also a problem in vacuum-sealing. Accordingly, each of a plurality of the discharge tube element of metal material is connected to each other with an insulator interposed between both of the discharge tube elements. This insulator is interposed to form a discharge gap between both of them. In other words, each discharge tube element is used both as itself and as a discharge electrode, and the insulator is what has the function of securing a discharge gap between the discharge tube elements as the electrode. Moreover, an insulator also has a function of connecting hermetically discharge tube elements to each other. Yet further, when it is difficult to secure the hermeticity only with the insulator, sealing materials such as an O-ring are used.
Further, as a metal material which composes the discharge tube element, any material may be used in principle, but a material which produces less ill effect on substances to be processed, such as semiconductor devices, may be preferably used. In other words, the reason for this is that it cannot be said that there is not a possibility that a face of the inner circumference of a discharge tube element of metal material is subjected to sputtering by plasma, and the sputtered particles flow out from a plasma generating space as conductive material. In this sense, aluminum is preferable. Further, aluminum to which anodic oxidation treatment is applied is preferred to prevent corrosion by plasma. It is better to avoid using stainless steel, copper or the like as there is a possibility that these materials cause contamination. However, stainless steel, copper or the like may be used in some conditions.
Moreover, as an insulator for forming a discharging gap, any material may be used in principle, but an inorganic insulator of high purity, such as high purity alumina, sapphire, quartz and aluminum nitride, is preferable considering the heat resistance. Alternatively, polyimide, tetrafluoroethylene (Teflon (trademark of Du Pont Corp.)), polyethylene may also be used.
With respect to high-frequency power, its frequency is not particularly limited. In the concerned technical field, normally, high-frequency is taken as several hundreds kHz or greater. In a narrow sense, high-frequency is taken as 10 to 100 MHz in some cases. Also, a microwave is taken as 1 GHz or greater and, in a narrow sense, it is taken as about 103 to 104 MHz (2.45 GHz is well known in magnetron applications) in some cases. In the case of a microwave discharge, it is common to introduce a microwave using a waveguide. Though there are such technical notions generally accepted as described above, in the present invention, high-frequency power may include microwave power, and it is not necessary to be particular about the frequency if plasma ignition and plasma generation are possible.
A set of the discharge tube elements connected to each other as described above may be only a couple (the number of elements is two), two couples (the number of elements is three) or more (the number of elements is four or more).
In either case, by connecting each of a plurality of discharge tube elements of metal material to each other as described above, a discharge tube is constructed, and, in this discharge tube, almost the whole thereof becomes a tube for forming a plasma generating space for flowing gas for plasma generation and generated plasma while containing them in a necessary space, and simultaneously discharge tube elements adjacent to the direction of the tube axis serve as a pair of electrodes. That is, the discharge tube elements, respectively, have two functions different from those of each other. Consequently, it becomes possible to perform plasma ignition at the discharge gap between both discharge tube elements by applying high-frequency power to adjacent discharge tube elements.
Operation in accordance with the first embodiment of the present invention is as follows. Since a discharge tube consisting of a combination of a plurality of discharge tube elements of metal material is substantially formed of metal material, its physical strength is high compared with that of an insulating discharge tube in the case of prior art. The thermal strength is also high compared with that of an insulating discharge tube. Cooling properties are also good. Accordingly, even in a condition of being exposed to plasma of a high temperature, thermal deterioration is less. Further, the discharge tube element is not broken as distinct from an insulating discharge tube even though a large pressure is applied to the wall of the discharge tube element by a pressure difference since the interior of the discharge tube element is evacuated of air. Moreover, even when highly reactive plasma is generated, a discharge tube of metal material experiences less corrosion compared with an insulating discharge tube in the case of prior art. Yet further, this results in the extension of life and leads to reduction of maintenance. Still further, since plasma ignition is performed at a narrow gap, such as a discharge gap by applying high-frequency power, plasma ignition performance is good and plasma generation can be favorably realized. In addition, the discharge tube which is substantially of metal material almost as a whole becomes low in initial cost compared with a conventional insulating discharge tube, as well.
A plasma generating apparatus of a second embodiment of the present invention, has as a plurality of foregoing discharge tube elements of metal material, includes a first discharge tube element positioned at the middle, a second discharge tube element having a gas induction port for plasma and a third discharge tube element having a plasma discharge port, wherein these respective discharge tube elements are connected hermetically to each other in the direction of a tube axis. This corresponds to a more specific description of the first embodiment of the present invention described above. Here, any aspects of a gas introduction port for plasma and a plasma discharge port may be used. The discharge tube to be assembled from three sections. A discharge tube is divided into three discharge tube elements by being divided at two locations in the direction of a tube axis. There are a first discharge tube element at the middle, a second discharge tube element on the upstream side of a flow path, which has a gas introduction port for plasma, and a third discharge tube element on the downstream side of a flow path, which has a plasma discharge port.
An advantage the discharge tube assembled from divided three sections is as follows. Functions of a discharge tube can be separated into three functions, that is, a function introducing gas for plasma generation from the outside, ionizing gas to plasma by igniting gas in a plasma generating space, and supplying the generated plasma to a plasma processing chamber. Corresponding to the functions, three discharge tube elements are prepared, and thereby it becomes possible to adopt a constitution suitable for each function. In addition, discharge gaps are located at two positions of the upstream and the downstream of a flow path, and the performance of plasma ignition becomes high when it is arranged to perform plasma ignition at both discharge gaps. Plasma ignition is not necessarily required to be performed at both discharge gaps, and it might be performed at either discharge gap.
A plasma generating apparatus of a third embodiment of the present invention the second embodiment of the present invention described above, wherein, as the relation of connection between the foregoing first to third discharge tube elements and a high-frequency power supply supplying the foregoing high-frequency power, a high voltage terminal of the foregoing high-frequency power supply is connected to the foregoing middle first discharge tube element and a grounding terminal of the foregoing high-frequency power supply is connected to the foregoing second and third discharge tube elements on both sides. This corresponds to a more specific description of the second embodiment of the present invention described above.
An upstream second discharge tube element communicates with an external gas piping and gas cylinder through its gas introduction port for plasma. Also, a downstream third discharge tube element communicates with an external plasma processing chamber at its plasma discharge port. It is a problem from a safety standpoint to apply a high voltage terminal of the high-frequency power supply to a gas piping and a gas cylinder. And, it is also a problem from a safety standpoint to apply a high voltage terminal of the high-frequency power supply to a plasma processing chamber. In the latter, there are two safety problems of products and humans. Therefore, safety is secured by connecting a grounding terminal of the high-frequency power supply to both of the second and the third discharge tube elements on both sides. When a high voltage terminal of the high-frequency power supply is connected to the first discharge tube element at the middle, the first discharge tube element functions intrinsically as a common high voltage for both of a second discharge tube element and a third discharge tube element, and further ensures the respective levels of a high-frequency power (voltage) applied are similar.
A plasma generating apparatus of a fourth embodiment of the present invention, includes the first embodiment described above, has a plurality of foregoing discharge tube elements of metal material, includes a first discharge tube element positioned at the middle, an upstream second discharge tube element having a gas introduction port for plasma, a downstream third discharge tube element having a plasma discharge port, an intermediate fourth discharge tube element positioned between foregoing middle first discharge tube element and foregoing upstream second discharge tube element and an intermediate fifth discharge tube element positioned between the foregoing middle first discharge tube element and foregoing downstream third discharge tube element, wherein these respective discharge tube elements are connected hermetically to each other in the direction of a tube axis.
Since a mean free path of an electron becomes short as the gas pressure of a plasma generating space increases, it becomes difficult to spread plasma generation across the whole of the plasma generating space widely.
The distance traveled by a particle from one collision to next collision is referred to as a free path, and the average value thereof is referred to as a mean free path. A mean free path is inversely proportional to the gas pressure at a constant temperature. That is, the higher the gas pressure is, the shorter a mean free path of an electron is. Under the high gas pressure, since a mean free path of an electron becomes short, plasma generation will be limited to the vicinity of a discharge gap. Since, in a discharge tube assembled from three sections, the length of the middle first discharge tube element in the direction of tube axis becomes large in comparison with an inside diameter of the tube, a linkage between a plasma region centered on a discharge gap between the first discharge tube element and the second discharge tube element and a plasma region centered on a discharge gap between the first discharge tube element and the third discharge tube element is lost, and plasma results in not spreading across the whole of the plasma generating space of the discharge tube. When plasma regions are in conditions of losing such linkage, the efficiency of plasma generation becomes extremely low since there is a definite limit on an amount of plasma generated and an increase of plasma generation cannot be expected even though the power applied from a high-frequency power supply is increased.
Thus, the fourth embodiment of the present invention allows plasma generation efficiency to be high even in a condition of a high gas pressure which causes a mean free path of an electron to shorten. That is, a feature of the present invention is a discharge tube assembled from five sections. Compared with a discharge tube assembled from three sections, it can be said that by dividing the middle first discharge tube element further at the two points in the direction of a tube axis, two discharge tube elements of a fourth discharge tube element and a fifth discharge tube element are additionally made in addition to the first discharge tube element newly made and therefore a discharge tube is divided into five discharge tube elements on the whole including the upstream second discharge tube element and the downstream third discharge tube element. As a result, the distance between two discharge gaps is divided into three smaller sections in this invention. Dividing into three may include equal three sections and also unequal. Briefly speaking, the distance between discharge gaps adjacent to the direction of a tube axis is sufficiently shortened.
Accordingly, since gas for plasma generation is in a condition of a high gas pressure, a mean free path of an electron is short and four plasma regions, centered on the first to the fourth discharge gaps, respectively, in the direction of a tube axis is short in respective holding ranges of the direction of a tube axis, and plasma regions adjacent to the direction of a tube axis link together because the distance between discharge gaps is sufficiently shortened as described above. Therefore, a chain reaction of plasma generation is advanced and a plasma region is spread across the whole of the plasma generating space, and four plasma regions are combined into one. Even in a condition of a high gas pressure, an amount of plasma generated increases, and therefore it is possible to raise the efficiency of power supply and to enhance the efficiency of plasma generation.
A plasma generating apparatus of the fifth embodiment of the present invention includes the fourth embodiment of the present invention described above, wherein, as the relation of connection between the foregoing first to fifth discharge tube elements and a high-frequency power supply supplying the foregoing high-frequency power, a grounding terminal of the foregoing high-frequency power supply is connected to the foregoing middle first discharge tube element, the foregoing upstream second discharge tube element and the foregoing downstream third discharge tube element, and a high voltage terminal of the foregoing high-frequency power supply is connected to the foregoing intermediate fourth discharge tube element and the foregoing intermediate fifth discharge tube element.
Operation in accordance with the fifth embodiment of the present invention is as follows. Safety is secured by connecting a grounding terminal of a high-frequency power supply to an upstream second discharge tube element communicating with a external gas piping and gas cylinder through a gas induction port for plasma, a downstream third discharge tube element communicating with an external plasma processing chamber at a plasma discharge port and a middle first discharge tube element being separated by one intermediate element from the second and the third elements, respectively. The discharge tube element is constructed rationally in such a way that the levels of a high-frequency power (voltage) applied are similar to all of discharge gaps by connecting a high voltage terminal of the high-frequency power supply commonly to the intermediate fourth discharge tube element and the intermediate fifth discharge tube element.
A plasma generating apparatus of a sixth embodiment of the present invention includes the first embodiment described above, wherein the number of a plurality of foregoing discharge tube elements of metal material is an odd number.
Since a mean free path of an electron becomes short as gas pressure of a plasma generating space increases, it becomes difficult to spread plasma generation across the whole of the plasma generating space. Therefore, in the above fourth embodiment of the present invention, the distance between discharge gaps adjacent to the direction of a tube axis is shortened by making a discharge tube assembled from five sections and the efficiency of plasma generation is enhanced.
However, when the distance between discharge gaps is long even in a discharge tube assembled from five sections, the distance between discharge gaps may also be shortened by increasing further the number of division of the discharge tube to enhance the plasma generation efficiency.
In this case, as described later, it is preferred from a safety standpoint that the number of discharge tube elements is made an odd number by dividing a discharge tube into odd sections.
An advantage in accordance with the sixth embodiment of the present invention is as follows. Since the distance between discharge gaps can be set by finely adjusting, it is possible to set an optimal distance between discharge gaps depending on the conditions of plasma generation such as kinds of gas desired to be ionized to plasma, pressure of gas, applied electric power (voltage) and an applied frequency. And, when the number of division of a discharge tube is fixed, the distance between discharge gaps is lengthened in the case of a large size of a plasma generating apparatus, but, in accordance with the sixth embodiment of the present invention, an appropriate distance between discharge gaps may be set even in the case of a large size of a plasma generating apparatus.
A plasma generating apparatus of seventh embodiment of the present invention includes the sixth, wherein, as the relation of connection between the foregoing odd discharge tube element and a high-frequency power supply supplying the foregoing high-frequency power, a grounding terminal and a high voltage terminal of the foregoing high-frequency power supply are connected alternately to the foregoing odd discharge tube elements under the condition of connection that a grounding terminal of the foregoing high-frequency power supply is connected to an upstream second discharge tube element having a gas induction port for plasma and a downstream third discharge tube element having a plasma discharge port. This corresponds to a more specific description of the sixth.
When a grounding terminal and a high voltage terminal of a high-frequency power supply are connected alternately to a discharge tube element, in increasing the number of discharge tube elements, the levels applied to an upstream second discharge tube element and a downstream third discharge tube element vary depending on whether the number of discharge tube elements is an odd number or an even number.
That is, the levels applied of an upstream second discharge tube element differs from that of a downstream third discharge tube element when the number of discharge tube elements is an even number. And, the levels applied of an upstream second discharge tube element is similar to that of a downstream third discharge tube element when the number of discharge tube elements is an odd number.
Here, as described above, it is preferred to connect a grounding terminal of a high-frequency power supply to an upstream second discharge tube element and a downstream third discharge tube element. Therefore, the number of discharge tube elements is an odd number and connection relations as described above are taken.
A plasma generating apparatus of an eighth embodiment of the present invention includes the first embodiment, wherein the number of foregoing discharge gaps capable of plasma ignition is an even number.
Making the number of discharge gaps an even number requires basically to divide a discharge tube into an odd number and to make the number of discharge tube elements an odd number (taking the number of discharge gaps as n, the number of discharge tube elements becomes n+1).
However, when the length of a discharge tube element in the direction of a tube axis is shorter than the distance between discharge gaps required to link plasma regions into one, a distance between discharge gaps which is capable of plasma ignition may be substantially lengthened not by connecting a grounding terminal and a high voltage terminal of a high-frequency power supply alternately to a plurality of discharge tube elements, but by connecting a high-frequency power supply to a discharge tube element in such a way that the levels of a high-frequency power (voltage) applied from a high-frequency power supply are the same on adjacent two or more discharge tube elements.
That is, there is a location in which a grounding terminal or a high voltage terminal of a high-frequency power supply is connected to adjacent two or more discharge tube elements sequentially, and a discharge gap located at this location will not conduct plasma ignition since the levels of a high-frequency power (voltage) applied to the adjacent discharge tube elements are the same.
Hence, the gap is not a discharge gap. Therefore, a distance between discharge gaps capable of plasma ignition may be substantially lengthened.
On the one hand, it is preferred from a safety standpoint to connect a grounding terminal of a high-frequency power supply to an upstream second discharge tube element and a downstream third discharge tube element.
Therefore, it becomes a criterion that the number of discharge gaps capable of plasma ignition is an even number.
In this case, even though a grounding terminal of the high-frequency power supply is connected to an upstream second discharge tube element having a gas induction port for plasma and a downstream third discharge tube element having a plasma discharge port, the number of discharge tube elements is not necessarily an odd number and may be an even number.
An advantage in accordance with the eighth embodiment of the present invention is as follows.
The distance between discharge gaps required to link plasma regions into one varies with conditions of plasma generation such as kinds of gas, pressure of gas, applied electric power (voltage) and an applied frequency. Consequently, since it is required to set the length of a discharge tube element in the direction of a tube axis in conformity with the conditions of plasma generation, commonality of a discharge tube element is difficult.
A discharge tube element has been fabricated, of which the length in the direction of a tube axis is shorter than the distance between discharge gaps required to link plasma regions into one, and by contriving the order of connection of a grounding terminal or a high voltage terminal of a high-frequency power supply to be connected to each of a plurality of discharge tube elements to provide a distance between discharge gaps in conformity with the conditions of plasma generation, a distance between discharge gaps capable of plasma ignition may be set. Thus, commonality of a discharge tube element becomes possible.
Particularly, this manner is effective when a large number of discharge tube elements are used in the case of a large size of a plasma generating apparatus.
A plasma generating apparatus of a ninth embodiment of the present invention includes the first to the eighth embodiments, wherein connection between the foregoing discharge tube elements to each other lets flanges, which are directed radially outward, integrally conjoined to each discharge tube element to oppose to each other and the foregoing insulator is interposed between the opposite flanges.
Operation in accordance with the ninth embodiment of the present invention is as follows. Though a discharge tube is divided in the direction of a tube axis and a discharge tube is constructed by connecting the discharge tube elements resulting from that division to each other, a discharge tube is brought into an ultra-low pressure condition, even though temporarily, by evacuating. Accordingly, dividing a discharge tube becomes a significant problem on vacuum-sealing. A performance of vacuum-sealing has to be surely secured also in dividing. There is a structure of flange for this purpose. In each discharge tube element, a flange extending radially outward is integrally conjoined to the corresponding location connecting each discharge tube element to another discharge tube element. A dimension of the circumferential direction differs from that of the direction of a tube axis in the directivity. Accordingly, it is possible to construct a flange with sufficient size and area corresponding to the requirements. The requirements are the performance of vacuum-sealing and the strength of connection. A discharge tube element is connected to each other at the flanges opposed to each other with an insulator interposed between the flanges. When an insulator is supported by sandwiching it between the flanges with required area, performance of vacuum-sealing may be sufficiently secured. In addition, an arrangement of a seal material such as an O-ring also becomes easy. And, it also becomes possible to make the strength of connection sufficiently high. And, a discharge gap is formed by interposing an insulator, and, by setting sufficient area for the insulator interposed, it is possible to make the dimensional accuracy of a discharge gap sufficiently high since the deformation due to the external stress becomes less. This is extremely significant for plasma ignition.
In a plasma generating apparatus of a tenth embodiment of the present invention includes the above first to ninth embodiments, the foregoing respective discharge tube elements are provided with cooling medium flow paths separately each other.
Operation in accordance with the tenth embodiment of the present invention is as follows. A discharge tube is excellent in heat resistance compared with that of an insulating discharge tube in the case of prior art since respective discharge tube elements composing the discharge tube are formed of metal material. Even then, since the generated plasma is at a high temperature, it is preferred to cool a discharge tube. Though it is conceivable to cool with air, it is more effective to cool using a liquid cooling medium. Considering leakage at a portion of a connection between discharge tube elements, it is preferred to provide an individual cooling medium flow path for each discharge tube element. As for leakage at a circumferential surface of a discharge tube, since this discharge tube is formed of metal material and resistant to failures, such as cracks and fractures resulting from thermal detrioration as distinct from prior art using an insulating discharge tube, leakage of cooling medium at a circumferential surface of a discharge tube does not occur in principle. Thereby, a discharge tube composed of a plurality of elements allows preventing the leakage of cooling medium. And, with respect to kinds of cooling medium, it has been thought in the prior art that a substance which does not absorb high-frequency power as far as possible, for example, an expensive one such as deionized water, is preferable, but, in the present invention, there is not such a limit, and relatively inexpensive cooling medium available may be used and this reduces the running cost.
A plasma generating apparatus of an eleventh embodiment of the present invention includes the above first to ninth embodiments of the present invention, wherein a part of a plurality of the foregoing discharge tube elements is provided with cooling medium flow paths and the rest of the discharge tube elements are cooled with air.
Operation in accordance with the eleventh embodiment of the present invention is as follows. As for cooling of each discharge tube element, it is preferred to provide an individual cooling medium flow path for each discharge tube element like the above tenth aspect of the present invention, but when it is possible to fabricate a discharge tube element which can stand the heat of plasma generated even though a part of discharge tube elements is cooled with air without providing all of respective discharge tube elements with cooling medium flow, it becomes possible to provide only a part of discharge tube elements with cool medium flow paths and cooling the rest discharge tube elements with air. Such a constitution reduces the number of cooling medium flow paths, the initial cost and the running cost of a plasma generating apparatus including the reduction of equipment of piping system and consumption of a flowing cooling medium and simplifies constitution of a discharge tube.
A plasma generating apparatus of a twelfth embodiment of the present invention includes the first to eleventh embodiments of the present invention described above, wherein, in a relation between the foregoing discharge gap and the foregoing insulator, a gap between the foregoing discharge tube elements, connecting to form the foregoing discharge gap, is formed by connecting between a part along the direction perpendicular to the direction of a tube axis of the foregoing discharge tube element and a part along the direction of a tube axis of the foregoing discharge tube element.
Operation in accordance with the twelfth embodiment of the present invention is as follows. Though a discharge gap is constructed by connecting a discharge tube element to each other in a state of supporting an insulator by sandwiching it, a face of the inner circumference of the insulator is communicated with an inner plasma generating space through the discharge gap. Accordingly, in a condition of containing a conductive substance in plasma, there is possibility that the conductive substance adheres to a face of the inner circumference of an insulator across a discharge gap. An insulator has a role isolating a pair of discharge tube elements and is supported by being sandwiched between a pair of discharge tube elements. Accordingly, a face of the inner circumference of an insulator is physically and mechanically connected to both discharge tube elements apart from electrically. Accordingly, when the conductive substance adheres to a face of the inner circumference of the insulator, there is a possibility that both discharge tube elements of metal material are short circuited through the conductive substance. Occurrence of a short circuit obstructs plasma ignition itself, and reliability of plasma generation becomes a problem.
A gap of both discharge tube elements connected to a discharge gap is bent. The manner of bending may be inflecting or curving. Briefly speaking, it might be well that a gap takes aspects of connecting between a part along the direction perpendicular to the direction of a tube axis and a part along the direction of a tube axis of a discharge tube elements, respectively. Each part may be connected to each other orthogonally or through the medium of a curved portion with a certain curvature or tapered portion. Even if there is an event that conductive substance in a plasma generating space plunges into the rear gap of a discharge gap due to collision against another particle, there is a tendency that the path is along the direction perpendicular to the direction of a tube axis. If a conductive substance adheres at the rear end of the rear gap of a discharge gap, a part along the direction of a tube axis still remains an insulator. A flying conductive substance resists colliding against the part along the direction of a tube axis, the adhesion and deposition are retarded. Thus, by synergy between that there is a spacial allowance and resistance to collision, it is possible to prevent a short circuit through a conductive substance and reliability of a plasma generating apparatus may be enhanced.
A plasma generating apparatus of a thirteenth embodiment of the present invention includes the above first to twelfth embodiments of the present invention, wherein an insulator interposed between foregoing discharge tube elements is separated into an outer body portion for vacuum-sealing and an inner protecting portion exposed to plasma.
Operation in accordance with the thirteenth embodiment of the present invention is as follows. Since an insulator for forming a discharge gap is exposed directly to the plasma of a high temperature and the vicinity of the insulator is not cooled directly, it is subject to local overheating and apt to cause the thermal deformation, and there is a possibility that the thermal deformation produces the failure such as cracks and fractures in the insulator to cause the vacuum break. Thus, an insulator is separated into an inner protecting portion and an outer body portion, and a body portion is provided with a function of vacuum-sealing, and a protecting portion is made a portion for allowing the crack and fracture. Even if cracks and fractures develop due to an exposure to plasma, the protecting portion prevents the crack and fracture of the protecting portion from communicating with the body portion. That is, the protecting portion is made to bear the role of a breakwater against thermal deterioration, made victims in thermal deterioration and made to protect the rear body portion. Further a body portion and a protecting portion may be spatially separated or contacted. It is preferred for limiting the communicating to keep them separated a little. If it is possible to substantially prevent failure such as the crack and fracture from communicating, a substance which is integrally connected to a body portion through the medium of a narrow part may also be used. Since the body portions are free from thermal deterioration, the performance of vacuum-sealing is secured over an extended time period and the reliability of a plasma generating apparatus may be enhanced.